Nutrient digestibility, nitrogen excretion, and milk production of mid-lactation Jersey × Friesian cows fed diets containing different proportions of rumen-undegradable protein

Abstract The present study was planned to test the hypothesis that feeding lactating dairy cattle with varying levels of rumen-undegradable protein (RUP) can enhance protein utilization, milk production, milk protein, and nitrogen (N) excretion. Forty mid-lactating crossbred (Jersey × Friesian) cattle were randomly divided into four groups. Four treatment diets were formulated to contain 30%, 40%, 50%, and 60% RUP of crude protein. Dry matter (DM) and crude protein intakes were significantly reduced with increasing dietary RUP levels. Crude protein digestibility increased linearly with incremental increases in dietary RUP levels. Cattle fed 60% RUP showed a linear decrease in N intake compared to that in the other groups. A linear decrease in urinary N and linear increases in net N, milk N, and N-use efficiency were observed with increasing dietary RUP levels. Actual milk, energy-corrected milk, and 4% fat-corrected milk yields (kg/day) increased linearly with an increasing degradability of protein. However, milk protein, solids not fat and total solids, as well as the yields of protein, fat, and lactose, showed significant increases with increased RUP supplementation. Collectively, the results indicate that formulating dairy cow diets to contain 60% RUP results in better lactating performance and N-use efficiency and lower N excretion.

摘要 本研究旨在验证下述假说：给泌乳奶牛饲喂不同水平的瘤胃非降解蛋白（rumen-undegradable protein, RUP），可提升蛋白质利用率、产奶量、乳蛋白含量及氮（nitrogen, N）排泄量。选取40头泌乳中期的娟姗牛×弗里斯兰（Jersey × Friesian）杂交奶牛，随机分为4组。配制4种试验日粮，其粗蛋白中RUP占比分别为30%、40%、50%及60%。随着日粮RUP水平升高，干物质（dry matter, DM）采食量与粗蛋白采食量显著降低。粗蛋白消化率随日粮RUP水平升高呈线性升高。饲喂60% RUP日粮的奶牛氮采食量较其余各组呈线性下降。随着日粮RUP水平升高，尿氮排出量呈线性下降，而净氮、乳氮及氮利用效率呈线性升高。实际产奶量、能量校正乳及4%乳脂校正乳产量（kg/天）随蛋白质降解率升高呈线性提升。但乳蛋白、非脂固形物与总固形物含量，以及蛋白、脂肪和乳糖产量均随RUP添加量提升出现显著升高。综合来看，本研究结果表明，将奶牛日粮配置为含60% RUP的配方，可获得更优的泌乳性能与氮利用效率，并降低氮排泄量。